The present invention relates generally to an anti-skid brake control system in which wheel acceleration is derived in order to detect braking conditions as part of braking control. More particularly, the invention relates to a method and system for deriving wheel acceleration which compensates for time lag in the derivation of wheel acceleration and deceleration.
In general, wheel slippage during braking is best maintained in a predetermined range so that skidding or locking of the wheels can be reliably prevented and that the braking characteristics of the vehicle can be optimized. As is well known, in anti-skid control, the braking pressure to be applied to wheel cylinders is to be so adjusted that the peripheral speed of the wheels during braking is held to a given range, e.g. 80%, of the vehicle. Such practice has been believed to be effective especially when road conditions and other factors are taken into consideration. Through out the accompanied disclosure, the ratio of wheel peripheral speed to vehicle speed will be referred to as "slip rate" or "slip ratio".
In the anti-skid control, working fluid pressure in a wheel cylinder in a hydraulic brake circuit is associated with a pressure control valve which is operative in an application mode for increasing the fluid pressure in the wheel cylinder, in an releasing mode for decreasing the fluid pressure in the wheel cylinder and in a holding mode for holding the fluid pressure in the wheel cylinder at a substantially constant level. One of the foregoing operational mode is selected depending upon the braking condition derived based on a wheel speed, a wheel acceleration, a slip rate and so forth.
In U.S. Pat. No. 4,408,290, issued on Oct. 4, 1983 to the common inventor and commonly assigned to a common assignee, discloses a method and system for sampling input time data for use in calculation of acceleration and deceleration. In the disclosure of the applicant's prior invention, an acceleration sensor acts on the variable-frequency pulses of a speed sensor signal to recognize any variation of the pulse period thereof and to produce an output indicative of the magnitude of the detected variation to within a fixed degree of accuracy. The durations of groups of pulses are held to within a fixed range by adjusting the number of pulses in each group. The duration of groups of pulses are measured with reference to a fixed-frequency clock pulse signal and the measurement periods of successive groups of equal numbers of pulses are compared. If the difference between pulse group periods is zero or less than a predetermined value, the number of pulses in each group is increased in order to increase the total number of clock pulses during the measurement interval. The number of pulses per group is increased until the difference between measured periods exceeds the predetermined value or until the number of pulses per group reaches a predetermined maximum. Acceleration data calculation and memory control procedure are designed to take into account the variation of the number of pulse per group.
The wheel speed is calculated periodically after the pulse interval or duration of the grouped sensor pulses becomes greater than a predetermined value. A slip rate is periodically derived from the derived wheel speed. On the other hand, the wheel acceleration is calculated only when the difference between the sensor pulse intervals or the durations of successive sensor pulse groups becomes larger than a given value. Therefore, wheel acceleration may be derived later than the wheel speed and the slip rate. Since the operation mode is derived periodically corresponding to calculation timing of the wheel speed and slip rate, the delay in deriving the wheel acceleration value can cause errors in selecting the operation mode.
In particularly significant cases, one of the operational modes in a predetermined skid control pattern may be skipped, i.e. not performed during one cycle of the skid control operation. For example, it is possible for the holding mode to be selected immediately after having selected the holding mode, i.e. to skip either a release mode or an application mode. In such case, if it is an application mode which is skipped, and thus the fluid pressure in the wheel cylinder is held at a reduced value, wheel speed will increase non-optimally, resulting in an unnecessarily prolonged braking distance. On the other hand, if a release mode period is skipped, the fluid pressure in the wheel cylinder will be held at an excessively high value, increasing the slip rate non-optimally, which may result in a skid.
Such danger is caused when one period of a scheduled operational mode cycle is skipped.